After the cutscene, get in the car and drive over to the waypoint near the jewelry store. Franklin will exit the car and he's tasked with tossing the gas grenade into the ventilation system.

Go to the waypoint on the rooftop and then toss a gas grenade into the vent across from Frank's direction. Aim a little bit lower than the ventilation hole or otherwise Franklin will overthrow the grenade.

Depending on the skills of your gunmen, if their skillset is low, one of them will crash before entering the tunnels (see screencap above), which reduces your cash intake by 30%. If that happens, pick up his dropped duffel bag by driving over it. If their skill set is high, they will continue into the tunnels with no money lost.

Keep following the guys onto the freeway and down into to the tunnels. Swerve thru the tunnels to emerge out into the LS River where control switches to Michael in the van.